<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>iframe</title>
</head>

<body>
  <div id="root"></div>
  <iframe id="ifm" src="/ajax" frameborder="1" style="display: none;"></iframe>
  <script>
    setInterval(() => {
      ifm.src = '/ajax?vt=' + Math.random()
    }, 3000)
  </script>
</body>

</html>